If you get a prompt for your master password or for a proxy password, then Firefox is not able to check for updates until you receive that prompt and enter the correct password. As a workaround, you can periodically check for updates manually after you've entered your password.

Incorrect update URL setting

Firefox may be set to use an incorrect address when checking for updates.

The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!I accept the risk! to continue to the about:config page.

Search for the preference app.update.url.

Right-clickHold down the Ctrl key while you clickapp.update.url and click Reset.

If Reset is disabled, Firefox is already set to use the correct update address, so this isn't your problem.

Now that Firefox is configured with the correct address, it may be able to update without error.

Cookies - Incorrectly set or Cannot be set

Firefox may not be able to set or read cookies when it checks for updates. Make sure your Firefox is able set or access cookies, especially when using Stealther and/or other Privacy and Security add-ons.

Find a cookie beginning with the name aus2. If it exists, select the site and click Remove Cookie(s).

Eset security software

If you use Eset security software, it is known to cause this error because of the way it checks secure connections. For instructions on how to fix the problem, read this Eset article.

Upgrading manually

If nothing else works, you can always download the latest version of Firefox from http://www.getfirefox.com and install it over top of your existing version. You won't lose your bookmarks or other settings. To prevent the error dialog from coming up, you can turn off automatic updates.